Part 24 - Boot Sector Basics [Part 7]

For a complete table of contents of all the lessons please click below as it will give you a brief of each lesson in addition to the topics it will cover. https://github.com/mytechnotalent/Reverse-Engineering-Tutorial

We need to discuss memory at this point. Before we can discuss setting up a simple stack in our bootloader we must understand how memory is allocated in the bootsector.

1)0x0 = Interrupt Vector Table - This is where our interrupt table exists at the very base of memory. This is where all of our interrupt calls exist.

2)0x400 = BIOS Data Area - This stores variables about the state of the bootable device.

3)0x7c00 = Loaded Boot Sector - This has our machine code that will be loaded into RAM by the bootloader firmware (note: firmware is simply code that runs before an OS runs like what we are doing).

4)0x7e00 = Free - This is your stack area that you can develop in.

5)0x9fc00 = Extended BIOS Data Area - Holds data from disk track buffers and other connected devices as remember there is no file system as of yet.

6)0xa0000 = Video Memory - BIOS maps your video memory here at boot.

7)0xc0000 = BIOS - Where BIOS officially resides.

8)0x100000 = Free - Additional space you can develop in.

This is critical that you understand how memory is laid out at boot. In our next lesson we will create a simple stack at 0x7e00.